SNIS175A March   2013  – January 2015 LMT88

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Revision History
  5. 5 Pin Configuration and Functions
  6. 6 Specifications
    1. 6.1 Absolute Maximum Ratings
    2. 6.2 ESD Ratings
    3. 6.3 Recommended Operating Conditions
    4. 6.4 Thermal Information
    5. 6.5 Electrical Characteristics
    6. 6.6 Typical Characteristics
  7. 7 Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1 LMT88 Transfer Function
    4. 7.4 Device Functional Modes
  8. 8 Application and Implementation
    1. 8.1 Application Information
      1. 8.1.1 Capacitive Loads
    2. 8.2 Typical Applications
      1. 8.2.1 Full-Range Centigrade Temperature Sensor
        1. 8.2.1.1 Design Requirements
        2. 8.2.1.2 Detailed Design Procedure
        3. 8.2.1.3 Application Curve
      2. 8.2.2 Centigrade Thermostat
        1. 8.2.2.1 Design Requirements
        2. 8.2.2.2 Detailed Design Procedure
        3. 8.2.2.3 Application Curve
    3. 8.3 System Examples
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Example
    3. 10.3 Thermal Considerations
  11. 11Device and Documentation Support
    1. 11.1 Trademarks
    2. 11.2 Electrostatic Discharge Caution
    3. 11.3 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

Package Options

Mechanical Data (Package|Pins)
Thermal pad, mechanical data (Package|Pins)
Orderable Information

8 Application and Implementation

NOTE

Information in the following applications sections is not part of the TI component specification, and TI does not warrant its accuracy or completeness. TI’s customers are responsible for determining suitability of components for their purposes. Customers should validate and test their design implementation to confirm system functionality.

8.1 Application Information

The LMT88 has very low supply current and a wide supply range therefore it can easily be driven by a battery as shown in Figure 4.

8.1.1 Capacitive Loads

The LMT88 handles capacitive loading well. Without any precautions, the LMT88 can drive any capacitive load less than 300 pF, as shown in Figure 2. Over the specified temperature range the LMT88 has a maximum output impedance of 160 Ω. In an extremely noisy environment it may be necessary to add some filtering to minimize noise pickup. TI recommends adding 0.1 μF from V+ to GND to bypass the power supply voltage, as shown in . In a noisy environment it may even be necessary to add a capacitor from the output to ground with a series resistor as shown in . A 1-μF output capacitor with the 160-Ω maximum output impedance and a 200-Ω series resistor will form a 442-Hz lowpass filter. Because the thermal time constant of the LMT88 is much slower, the overall response time of the LMT88 will not be significantly affected.

In situations where a transient load current is placed on the circuit output the series resistance value may be increased to compensate for any ringing that may be observed.

No_decoupling_required_for_capacitive_loads_less_nis175.gifFigure 2. LMT88 No Decoupling Required for Capacitive Loads Less Than 300 pF

Table 2. Capacitive Loading Isolation

Minimum R (Ω) C (µF)
200 1
470 0.1
680 0.01
1 k 0.001

filter_for_noisy_environment_capacitive_loading_greater_nis175.gifFigure 3. LMT88 With Filter for Noisy Environment and Capacitive Loading Greater Than 300 pF

NOTE

Either placement of resistor as shown in Figure 2 and Figure 3 is just as effective.

8.2 Typical Applications

8.2.1 Full-Range Centigrade Temperature Sensor

typ_app_nis175.gifFigure 4. Full-Range Celsius (Centigrade) Temperature Sensor (−55°C to 130°C)

8.2.1.1 Design Requirements

Because the LMT88 is a simple temperature sensor that provides an analog output, design requirements related to layout are important, refer to Layout for detailed description.

8.2.1.2 Detailed Design Procedure

The LMT88 output follows Equation 5.

Equation 5. VO = (−3.88×10−6×T2) + (−1.15×10−2×T) + 1.8639

Solving for T:

Equation 6. equation_2A_nis175.gif

where

  • T is temperature, and VO is the measured output voltage of the LMT88. Equation 6 is the most accurate equation that can be used to calculate the temperature of the LMT88.

An alternative to the quadratic equation a second order transfer function can be determined using "least squares" method:

Equation 7. T = (−2.3654×VO2) + (−78.154×VO ) + 153.857

where

  • T is temperature express in °C and VO is the output voltage expressed in volts.

A linear transfer function can be used over a limited temperature range by calculating a slope and offset that give best results over that range. A linear transfer function can be calculated from the parabolic transfer function of the LMT88. The slope of the linear transfer function can be calculated using the following equation:

Equation 8. m = −7.76 × 10−6× T − 0.0115,

where

  • T is the middle of the temperature range of interest and m is in V/°C. For example for the temperature range of TMIN = −30 to TMAX = 100°C:
Equation 9. T = 35°C

and

Equation 10. m = −11.77 mV/°C

The offset of the linear transfer function can be calculated using the following equation:

Equation 11. b = (VOP(TMAX) + VOP(T) − m × (TMAX+T))/2

where

  • VOP(TMAX) is the calculated output voltage at TMAX using the parabolic transfer function for VO.
  • VOP(T) is the calculated output voltage at T using the parabolic transfer function for VO.

Using this procedure, the best fit linear transfer function for many popular temperature ranges was calculated in Table 3. As shown in Table 3, the error that is introduced by the linear transfer function increases with wider temperature ranges.

Table 3. First Order Equations Optimized for Different Temperature Ranges

TEMPERATURE RANGE LINEAR EQUATION MAXIMUM DEVIATION OF LINEAR EQUATION
FROM PARABOLIC EQUATION (°C)
Tmin (°C) Tmax (°C)
−55 130 VO = −11.79 mV/°C × T + 1.8528 V ±1.41
−40 110 VO = −11.77 mV/°C × T + 1.8577 V ±0.93
−30 100 VO = −11.77 mV/°C × T + 1.8605 V ±0.70
-40 85 VO = −11.67 mV/°C × T + 1.8583 V ±0.65
−10 65 VO = −11.71 mV/°C × T + 1.8641 V ±0.23
35 45 VO = −11.81 mV/°C × T + 1.8701 V ±0.004
20 30 VO = –11.69 mV/°C × T + 1.8663 V ±0.004

8.2.1.3 Application Curve

output_voltage_vs_temperature_nis175.gifFigure 5. Output Voltage vs Temperature

8.2.2 Centigrade Thermostat

centigrade_thermostat_Circuit_nis175.gifFigure 6. Centigrade Thermostat

8.2.2.1 Design Requirements

A simple thermostat can be created by using a reference (LM4040) and a comparator (LM7211) as shown in Figure 6.

8.2.2.2 Detailed Design Procedure

The threshold values can be calculated using the following equations.

Equation 12. centigrade_thermostat_VT1_EQ_nis175.gif
Equation 13. centigrade_thermostat_VT2_EQ_nis175.gif

8.2.2.3 Application Curve

centigrade_thermostat_Curve_nis175.gifFigure 7. Thermostat Output Waveform

8.3 System Examples

The LMT88 draws very little power therefore it can simply be shutdown by driving its supply pin with the output of an logic gate as shown in Figure 8.

conserving_power_dissipation_with_shutdown_nis175.gifFigure 8. Conserving Power Dissipation With Shutdown

Most CMOS ADCs found in ASICs have a sampled data comparator input structure that is notorious for causing problems for analog output devices such as the LMT88 and many operational amplifiers. The cause of this difficulty is the requirement of instantaneous charge of the input sampling capacitor in the ADC. This requirement is easily accommodated by the addition of a capacitor. Because not all ADCs have identical input stages, the charge requirements will vary necessitating a different value of compensating capacitor. This ADC is shown as an example only. If a digital output temperature is required, refer to devices such as the LM74.

suggested_conn_to_a_sampling_analog_digital_converter_nis175.gifFigure 9. Suggested Connection to a Sampling Analog-to-Digital Converter Input Stage